One reason that this is important is because you never know when the income you are relying on may dry up. This is most exceptionally true for freelancers; you never know when a company may cancel your contract or decide not to renew it.
Take for example myself, I currently work for four different sources online and have 2-3 that I do the minimum for as a backup. This will ensure that, should any one of my main ones dry up, I will have another to take its place. It may seem like that is a bit of overkill on work to do, but I have experienced from many others that it can be a bad thing to rely too much on one source. Doing this also helps to ensure that should one of my sources have a dry spell, I can just pick up some extra work on another.
Another thing that I make sure about all my work is that I can earn a certain amount with each of them. This will help to ensure that I can maintain my same income without having to put in an additional amount of hours.
Another benefit of having more than one source of income while working at home is, should an emergency expense come up, it is within my ability to pick up extra work on one of my extra sources. This will allow me to keep my stress levels lower, knowing that I can easily acquire the needed amount of money.